1. Field of the Invention
The present invention relates to a process for producing active carbon used for polarizable electrodes for an electric double layer capacitor making use of an electrolytic organic solvent solution.
2. Description of the Background Art
Since an electric double layer capacitor has a large capacity of the farad level and is also excellent in charge and discharge cycle properties, it is used in applications such as back up power supplies for electronic equipment and batteries for automobiles.
In this electric double layer capacitor, for example, as illustrated in FIG. 3, a pair of polarizable electrodes 1, 1 composed of active carbon are arranged in an opposed relation to each other through a separator 2, and the polarizable electrodes 1, 1 are impregnated with an organic solvent solution of a tetraalkylammonium salt or the like as an electrolytic solution so as to function as positive and negative electrodes, respectively. In the electric double layer capacitor illustrated in FIG. 3, the polarizable electrodes 1, 1 opposed to each other through the separator 2 are contained in an aluminum container 3 which is closed with an aluminum lid 5 through a packing 4. In the above-described construction, the container 3 and the lid 5 come into separate contact with the polarizable electrodes 1, 1 in such a manner that the container 3 serves as a current collector member on the cathode side to the polarizable electrode 1, and the lid 5 serves as a current collector member on the anode side to the other polarizable electrode 1.
Although active carbon having fine pores is used for the polarizable electrodes for such an electric double layer capacitor, there is a demand for development of active carbon capable of more heightening the capacity density of the polarizable electrodes for the purpose of providing a smaller and lighter electric double layer capacitor having a greater capacity.
Therefore, various properties of active carbon have been investigated with a view toward providing active carbon capable of heightening the capacity density of the polarizable electrodes. For example, the hypothesis that "A capacity density per weight of active carbon in an electrode has an almost linear proportional relationship with a specific surface area of the active carbon, and the capacity of an electric double layer on the active carbon electrode is about constant without being affected by the kind of carbon and pore characteristics thereof" has been proposed (Electrochemistry, 59, No. 7, pp. 607-613, 1991).
When a theoretical value of the capacity of an electric double layer capacitor comprising polarizable electrodes making use of such active carbon is found from a value observed by a mercury electrode or the like as the capacity of the electric double layer of the active carbon and the specific surface area of the active carbon, however, the theoretical value does not consist with the found value of the capacity of the electric double layer capacitor.
For example, assuming that the capacity of the electric double layer capacitor so constructed that a pair of polarizable electrodes 1, 1 are arranged with a separator 2 held therebetween as illustrated in FIG. 3 is C.sub.0, and capacities of the polarizable electrodes 1, 1 are C.sub.1 and C.sub.2, the following equation is satisfied: EQU 1/C.sub.0 =1/C.sub.1 +1/C.sub.2 ( 1)
Here, the capacity of the electric double layer on the active carbon observed by the mercury electrode is about 20 .mu.F/cm.sup.2. Therefore, the capacity of the electric double layer on active carbon having a specific surface area of 1,500 m.sup.2 /cc amounts to:
20 (.mu.F/cm.sup.2).times.1,500 (m.sup.2 /cc)=300 (F/cc).
Then, 300 (F/cc) is substituted for C.sub.1 and C.sub.2 in the equation (1) to find C.sub.0. As a result, C.sub.0 amounts to 150 (F/cc). Since C.sub.0 is a capacity for two volumes of the polarizable electrode 1, the theoretical capacity of the electric double layer capacitor should amount to 75 F/cc obtained by dividing the C.sub.0 value by 2.
In reality, the capacity of the electric double layer capacitor amounts to only about 13 F/cc. Even when different kinds of active carbon having the same specific surface area are used, the capacities of the resultant electric double layer capacitors may be entirely different from each other in some cases.
Therefore, the present inventors paid attention to the fact that the above hypothesis is based on the specific surface area measured by nitrogen gas absorption in accordance with the BET method the analytical limit of which is about 10 angstroms, and attempted a special image analysis capable of analyzing even fine pores smaller than 10 angstroms by using an image through a transmission electron microscope in order to verify the above hypothesis, thereby investigating the relationship between capacity density and specific surface area as to various kinds of active carbon for electrodes different in specific surface area. As a result, the conclusion that there is no linear proportional relationship between them, and so factors affecting the capacity exist in some others was reached.
The present inventors carried out a further investigation on the basis of this finding. As a result, it was found that when a mode in the pore distribution of active carbon is controlled to a pore size suitable for adsorption of the electrolytic solution ions with the organic solvent solvated, active carbon excellent in capacity density per volume when used in polarizable electrodes for an electric double layer capacitor is provided. Such active carbon used for electrodes for the electric double layer capacitor was previously applied for patent (Japanese Patent Application No. 46912/1996). The active carbon described in the specification of this application is such that a carbonized product obtained by calcining a vinyl chloride resin is activated with an alkali in one stage by holding it for 1-20 hours at a temperature ranging from 400.degree. C. to 1,000.degree. C. The mode in its pore distribution is within a range of 10-20 angstroms, which are pore sizes suitable for adsorption of the electrolytic solution ions with the organic solvent solvated.
Incidentally, the mode means a value of a pore size which shows the highest relative frequency in the frequency distribution of pore sizes. Besides, the frequency distribution of pore sizes is found from a power spectrum obtained by converting the image of the active carbon through a transmission electron microscope into a binary image and subjecting this binary image to Fourier transformation.
According to such active carbon, a polarizable electrode high in electrode density and, in particular, excellent in capacity density per volume can be formed because the mode in its pore distribution falls within the above range, and an electric double layer capacitor high in energy density can be constructed by such polarizable electrodes.
However, the polarizable electrodes making use of the active carbon obtained by conducting the activation treatment under such conditions that the electrode density can be made high as described above involve a disadvantage that the resulting electric double layer capacitor is easy to lower the capacity by repeated charge and discharge.